Aim To develop a list of red flag clinical indicators for possible serious pathology masquerading as, or presenting alongside, neck related musculoskeletal disorders in the adult population. Background Musculoskeletal physiotherapists provide assessment and treatment for pain and functional impairments relating to musculoskeletal disorders, such as back and neck pain. In order to apply safe and effective treatment to these conditions it is vitally important that any underlying serious complaints have been excluded. Clinical indicators known as ‘red flags’ have been developed for diagnostic triage in back pain to help identify serious underlying conditions, such as cancer and infection. Red flags for serious pathology in neck pain or neck related pathology has not received the same level of attention as red flags in back pain. A literature review identified inconsistent evidence for clinical tests and clinical indicators for serious pathology in neck related musculoskeletal disorders. This presents a serious clinical challenge for musculoskeletal physiotherapists. Method A mixed method study design was developed involving: a) Qualitative descriptive method through Physiotherapy focus group; and, b) Three round Delphi survey method involving consultant neurologists and consultant neurosurgeons. The Delphi method involves combined qualitative and quantitative data phases. Thematic content analysis was used to analyse the qualitative data. A combined descriptive and inferential (non-parametric) statistical analysis was used to analyse the quantitative data. Kendall’s W (Kendall’s coefficient of concordance) was used to evaluate the level of consensus across all participants for the quantitative phase of the Delphi method. Findings A list of neck related red flag clinical indicators within five specific categories were developed: 1. progressive pain; 2. cancer, infection, trauma; 3. neurological deficit (spinal cord compromise); 4. headache (associated with neck pain/stiffness); 5. brainstem, cervical arterial and cranial nerve dysfunctions. An increase in Kendall’s W was demonstrated between Rounds 2 and 3 in four out of five categories, indicating an increase in consensus levels between participants. This process highlights the complexity of interpreting clinical features within musculoskeletal presentations.